The layer-by-layer (LBL) adsorption of oppositely-charged polyelectrolytes on surfaces has emerged as a simple, versatile, and inexpensive technique for the fabrication of thin multi-layer films, often with nanometer-scale control over the spatial distribution of ionized species within a film (Decher, G., Science, 277:1232-1237, 1997; Hammond P. T., Curr. Opin. Coll. Interface Sci., 3:32-39, 1998). First introduced by Decher in the early 90's (see Decher and Hong, Ber. Bunsenges. Phys. Chem. 95:1430, 1991 and Decher, Science, 277:1232, 1997), the LBL approach is based on electrostatic attractions between polyelectrolytes and oppositely charged surfaces. In the example shown in FIG. 1, a negatively charged substrate is first dipped in a polycation solution. Electrostatic attractions result in deposition of the polycation and a resulting reversal of surface charge (see FIG. 1, step 1). The positively charged substrate is then submerged in a polyanion solution, resulting in deposition of the polyanion and restoration of the negative charge on the surface (see FIG. 1, step 2). Repetition of these steps leads to the buildup of layers of alternating oppositely charged polyelectrolytes on the substrate surface. In addition to electrostatics, other factors and secondary interactions such as hydrophobicity, salt interactions, solvent quality, polymer concentrations, and deposition time may affect the multi-layer growth of the film (for a review of these factors, see Dubas and Schlenoff, Macromolecules 32.8153, 1999, the contents of which are incorporated herein by reference).
The array of materials available for LBL assembly is broad, including synthetic polyelectrolytes, conducting polymers, dyes, and metal colloids, as well as a variety of biological species such as proteins, viruses, and DNA. Applications as diverse as conductive and light-emitting films, biologically-active surfaces, selective membranes, patterned films, and hollow multi-layer structures underscore the potential of the LBL technique (for a review of applications, see Hammond, Curr. Opin. Coll. Interface Sci. 3:32, 1998, the contents of which are incorporated herein by reference).
Despite the incorporation of new functionality, there are relatively few examples of multi-layer thin films designed to release incorporated or encapsulated compounds. In particular, there remains a need in the art for thin film controlled release systems that function under physiological conditions.